There are three types of offpress color proofing systems in use. These are overlay, integral (or single sheet) and electronic (or digital) These proofing systems are described in "Principles of Color Proofing", M. H. Bruno, Gama Communications, Salem, N.H. 1986. In digital proofing systems, the individual color separation images are digitalized and either combined to produce a composite three-color soft proof on a color monitor, cathode ray tube, or video display terminal, or, as in the present invention, converted to a hard proof on a color proofing medium. In order to form a colored image on the medium, toner is added that adheres to the medium in defined areas One way of causing toner to adhere to the medium is to form a "tacky" image that will adhere to toner.
The use of light to cause "imagewise" formation of tackiness in a film is known as photo-tackification. Similarily, the use of infrared or near-infrared radiation to cause "imagewise" formation of tackiness is known as photothermal-tackification. The tacky image that is produced in the area exposed to infrared or near-infrared radiation can be made readily visible and read out by applying toner particles to the tacky surface. Toning of the tacky surface is possible using dry powders, for instance the Cromalin.RTM. toning agents, available from E. I. duPont de Nemours and Company, Wilmington, Delaware. The toner particles selectively adhere to tacky regions on a surface, thus providing negative-working materials. Such coatings can also be used on a substrate that is peelably attached, for example by lamination, to a toner-transfer foil, using such foils as Eurosprint.RTM. (registered in the United States as ColorTrax.RTM.) which are available from E. I DuPont de Nemours and Company. In these peel-apart elements, upon exposure of the element to infrared radiation, tackiness results on the substrate. Toner particles from the toner-transfer foil selectively adhere to the tacky regions generated on the substrate. When the substrate is separated from the foil, toner particles remain on the tacky regions of the substrate, thus providing the basis for negative-working color materials.
Examples of materials that directly result in tackiness upon irradiation with light or with infrared or near-infrared radiation are rare. Most negative working systems require a separate wet development step. The present composition achieves its tackiness because of the ability of a selected miscible polymer blend to undergo a photothermochemical change that results in a microphase separation of a tacky polymer from a non-tacky polymer, thereby eliminating the need for a separate wet development step.
R. E. Kellogg, H. Zwicker and J. Wall, "CREO/Crosfield Interface for IR Diode Thermal Imaging," IS&T Second Symposium on Electronic Prepress Technology and Color Proofing, Sep. 11-14, 1991 (Chicago, Ill.) disclose apparatus for thermal imaging using diode lasers.
U.S. Pat. No. 4,364,986 discloses a dye containing layer of a film-forming polymeric binder useful as optical filters and optical information recording elements.
U.S. Pat. No. 4,508,811 discloses a recording element having a pyrylium or thiopyrylium-squarylium dye layer and new pyrylium or thiopyrylium-squarylium dye compounds.
U.S. Pat. No. 3,563,738 discloses a reproduction material which combines a first tacky component and a light sensitive component which upon exposure to actinic radiation, changes the heat-activatability and thermal properties of the first component.
U.S. Pat. No. 4,356,252 teaches a photosensitive element useful for making color proofs comprising a support bearing a layer of negative-working tonable photoimaging composition comprising at least one organic polymeric binder, a photosensitizer which forms an acid upon exposure to actinic radiation, and at least one acetal compound wherein the polymeric binder is plasticized. U.S. Pat. No. 4,294,909 teaches a process for preparing negative tonable images comprising exposing imagewise a photosensitive element bearing a layer of negative-working tonable image composition as in U.S. Pat. No. 4,356,252. The principle involved in U.S. Pat. No. 4,356,252 and U.S. Pat. No. 4,294,909 is that a photogenerated acid decomposes a (solid) nonpolymeric acetal compound(s) and the resulting liquid products plasticize the binder so that T.sub.g is lowered and the binder becomes tacky enough to be toned. These patents differ from the present invention which is imagewise exposed by infrared or near infrared radiation and which achieves its tackiness as the result of the unmasking of a tacky polymer present in the composition, which has been previously masked by the presence of a nontacky polymer.
U.S. Pat. No. 4,346,162 discloses a nonsilver reproduction element for preparing negative tonable images which comprises a support bearing a layer of photosensitive composition comprising at least one thermoplastic binder and at least one thio compound
U.S. Pat. No. 5,019,549 uses "SQS" as an infrared absorber in a colorant transfer system.